The use of asymmetric cell division is central to generating cell type diversity during development. This process has been studied extensively using genetic and molecular approaches in both Caenorhabditis elegans and Drosophila melanogaster. However, both of these organisms are evolutionarily very high derived and may be relatively closely related. Thus it is difficult to define conserved core components of key regulatory pathways without comparison to a more distantly related and less derived experimental system For this purpose, we propose to initiate studies of a bilaterian organism more likely to resemble a primitive state, the annelid Platynereis dumerilii. This marine polychaete exhibits developmental features that link invertebrates and vertebrates, and provides impressive technical advantages. During early embryogenesis of Platynereis stereotypic asymmetric cell divisions generate distinct founder cell lineages, such as a mesodermal cell lineage called 4d. Thousands of synchronous embryonic stages can be easily obtained, the embryos are transparent and gene function can be reduced using RNA interference. To identify important regulators of early development, we propose to generate a P. dumerilii EST library and an in situ data base, and to examine gene requirements using both parental dsRNA interference and specific inhibitors of target genes. We also will employ microarray techniques to examine global changes in gene expression in mutant embryos. Our goals are to explore and exploit this new model system for studies of asymmetric cell division and nervous system development, and to use it as a comparative tool for defining the most highly conserved, functionally required core components of cellular machineries. Newly discovered and conserved loci will also be studied in C. elegans and Drosophila. [unreadable] [unreadable]